Tiny House Subfloor Framing, Insulation, and Sheathing

After leveling our tiny house trailer, subfloor construction had begun!  In order to gain more headspace in our lofts, we have a subfloor that is built within the trailer instead of on top.  While there are several cons (https://www.tinyhomebuilders.com/tiny-house-trailers/additional-information) to having a subfloor within the trailer, we have overcame these with our construction design. One of cons we overcame with a thermal bulkhead will be explained in our next post.

For the bottom of the subfloor, we decided to use ½” pressure treated sheathing instead of aluminum/steel flashing. Let’s be real- water is going to find its way somehow into your floor during construction. With metal flashing you’re just asking for your framing to be a bucket. You will add the sheathing after you constructed your framed box.

pressure treated sheathing on bottom of trailer.JPG

In preparation of your first frame, make sure to have the “crown” of your wood facing up so when weight bears down on it, the wood will “level” out. Also mark where your studs will go.

We had four framed boxes.  The back one was framed differently to allow for plumbing. Joists were framed every 16”. We used 2x6s since our frame was designed for them.  If you have a similar design like ours, you can measure the inner joists on the fly within the frame.


We used a chop saw to get everything to the dimensions we needed. A chop saw will become your best friend- buy or borrow one somehow.  Fortunate for us, my father-in-law has many contractor friends that are willing to let us borrow tools! Larry seen in most of these pictures is a jack-of-all-trades electrician that can literally build anything. It is safe to say that without him our build would be going a lot slower with many, many more mistakes! We are grateful for his help and expertise.  There is probably a father or father-in-law behind many tiny house builds!


We used a nail gun to secure all joists and for the bottom plywood. Ten penny (galvanized) nails (3”) attached the outside framing to the joists. Six penny (2 3/8”) nails were used to secure the sheathing.  Chalk lines can be used to keep track of where your joists are.

Our framed boxes required a full 4×8 plywood plus another cut to size.  We also notched out where bolts stick out in the frame that hold scissor jacks on the trailer.

Wood-to-metal tapping screws held the boxes in place within the frame.

tapping screws wood to metal joist box.JPG

To cut foam board insulation to size (quickly), we highly recommend a table saw with adjustable fence.  The only downside is you can’t easily cut foam boards by yourself as it more easily “binds” on the saw than wood.  If you happen to have 2-3 people working on the project though (like in our case), one person feeds, another person helps keep the cut portion along the fence, and another holds on the receiving end. You also will want to use a separate blade just for foam rather than use the same one you do for your wood.


We were able to accommodate 5 inches of foam board (two 2”& one 1” pieces) into our frame, giving us an R value of 25. We also used expandable window/door spray foam around our boxes & eventual plumbing.

Our box in the back was designed a little differently to accommodate plumbing.  We highly recommend creating a design to have all your plumbing on one side of your trailer (or at least close to).  Trust us, it’ll save hassle later.  Same goes for running propane lines, keep it to one side.  We chose driver’s side since that is more to RV code standards.

last box different for plumbing.JPG

Since our joists no longer followed across the metal frame, we created a doubled “header” secured with headlock screws.  These joists were attached with joist hangars.

Look for a thorough blog post in the future about plumbing. However we will briefly explain here what we did and why.  In a northern climate, we did not want any plumbing exposed/underneath the trailer like you typically see.  Our drain pipes are in an insulated subfloor. We plumbed for a flush toilet to have the option, but we will have a composting toilet above with urine divider attached to the drain line. We accommodated lines for a vent (air inlet), washer/dryer, shower with p trap accessibility, and bathroom faucet.


subfloor plumbing
General layout of our plumbing. Unlike most tiny house builds, our drain lines will be mainly in the subfloor, and pipes in the house will not be in external walls, only internal ones or knee walls.

Attach pipes with the slope you need. You can use PVC scrap as bracers until you have insulation place.


Before placing insulation, run a leak test. Plug your drain and run water from a hose through your lines.  Hopefully you won’t see leaking!

Place insulation where you can, including spray foam, but make sure it doesn’t mess with the slope you tried to achieve.

Remember you’re going to need to attach to these drain lines later.  We added a pipe that will be slightly above our AdvanTech once in place.

Once insulation is done, it’s time for AdvanTech (tongue and groove TNG plywood).  Keep in mind you lose part of your sheet from the tongue and groove coming together, so if you have a 28ft trailer like us, in the front & back you’ll need to compensate with using smaller sheets. We decided to start with our most difficult piece, one that had our plumbing and needed to hug the wheel wells.

We recommend chalk lines to know where your metal seems & wooden joists are for fastening.  We also used painter’s tape on our trailer to where metal seems & wood joists were at times.

We nailed AdvanTech into place.  We also used wood-to-metal tapping screws to secure.  When placing one sheet into another, you may need to use a hammer and wooden block to adjust it into place & make sure the sheet is flush on both sides of the trailer. Our last shorter piece was next to where we placed our first sheet. See that we used subfloor adhesive as well.

Lastly, we stapled a plastic sheet over the AdvanTech to serve as a vapor barrier. We placed scrap blocking on top of it so wind wouldn’t rip it away.  Good thing we finished the subfloor and vapor barrier because the very next morning was a torrential downpour!

Have any questions or want specifics about our subfloor build? Feel free to ask below!

mack and renee closeup during subfloor build.JPG
Feeling rejuvenated that our tiny house build is finally getting somewhere.

3 thoughts on “Tiny House Subfloor Framing, Insulation, and Sheathing

  1. Hi! Hopefully you’re still reading your comments even though it’s been awhile since this post. We’re in the process of building our tiny and like you said, most people don’t have their plumbing in the subfloor so there’s not a lot of info about it. We just sealed up our floor, drain pipes within, but now I’m second guessing our decision. How are things going with all that? Do you move it often (more than twice a year)? Any leaks or other issues? Thanks!


    1. Hi Lea!
      Yes, sealing up the floor can also feel like sealing up your fate! Before we covered up all our plumbing for good, we used a water hose and ran some water through the lines as a pretest to see if the drains would handle the way they should with the minimal elevation change in the lines. We haven’t ran into any problems yet, but we haven’t been living in our house just yet- we plan to move into it this spring. We been busy looking for a place to park it. We expect to only move the tiny house 2-3 times in its lifetime until we find a permanent spot.

      The only concern we had was any residue water in the lines freezing and cracking the drains during the winter. We added RV antifreeze to our drains (just a little will do) to prevent any problems until the drains are used normally. The other issue we ran into is realizing we barely gave ourselves enough room to install our toilet (but was able to with modifications), so you can only hope your floor layout is exactly where you plan it to be as there is no going back now. Good luck and we’d love to hear more about your house as it progresses! I hope to show updated photos of our completed plumbing and house at some point- I promise!! 🙂


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